Acrylic emulsions are known. They have been widely used in interior and exterior architectural and industrial coatings. Acrylic emulsions are usually prepared by emulsion polymerization of one or more alkyl acrylates or methacrylates. Acrylic monomers that contain a polar group are often used to stabilize the emulsion. These monomers include acrylic and methacrylic acids, and hydroxyalkyl acrylates and methacrylates. Introducing acid or hydroxyl functional groups into acrylic emulsions also makes them crosslinkable to form thermosetting coatings.
Usually, a combination of high-Tg (glass transition temperature) and low-Tg alkyl acrylates or methacrylates is used. Examples of low-Tg monomers are n-butyl acrylate (Tg: −54° C.), n-butyl methacrylate (Tg: 20° C.), and 2-ethylhexyl methacrylate (Tg: −10° C.). Methyl methacrylate (Tg: 100° C.) is the most commonly used high-Tg acrylic monomer. High-Tg monomers increase the coatings' gloss and hardness, while low-Tg monomers impart toughness and flexibility.
Styrene is also often incorporated into acrylic emulsions as a high-Tg monomer (Tg: 99° C.). Styrene is inexpensive. However, styrene-containing acrylic emulsions are often sensitive to UV (ultraviolet) light, and thus they have limited uses in exterior coatings.
Acrylic emulsions have replaced solvent-based acrylics in many areas, such as automotive base coats and industrial maintenance coatings. They are more environmentally friendly because reduced amounts of VOCs (volatile organic compounds) are used. However, acrylic emulsions and coatings therefrom are sensitive to moisture, which causes the coating surface to be cloudy and the coated metal surface to rust. Thus, new acrylic emulsions and latex coatings less sensitive to moisture and corrosion are needed.